1. Field of the Invention
The present invention relates to an observation system having a half mirror suited to a photographing apparatus such as a head mounted display or video camera which is designed to observe both video information, e.g., a liquid crystal display image, displayed on a display means and scenery image information within the same visual field, and a method of manufacturing the half mirror.
2. Description of the Related Art
A half mirror for splitting a light beam into a plurality of beams has been used in a conventional observation system for simultaneously observing pieces of image information, displayed in different directions, within the same visual field. For example, the optical system of an observation system mounted on a photographing apparatus such as a head mounted display or a video camera, which is designed to observe both image information, e.g., a liquid crystal display image, displayed on a display means and scenery image information within the same visual field, includes a half mirror for reflecting a liquid crystal display image toward the eyes of the observer and transmitting an external scenery image therethrough. As such a half mirror, for example, a dielectric multilayer film obtained by alternately stacking high-refractive-index films and low-refractive-index films, each consisting of a dielectric material, or a metal half mirror formed by sandwiching a metal film between dielectric films is often used.
According to conventional techniques of manufacturing half mirrors made of dielectric multilayer films or metal films, a large number of films are required for a dielectric multilayer film to obtain a relatively high reflectance in the visible region. In general, as the number of films to be formed increases, it becomes more difficult to manufacture a half mirror. In addition, the manufacturing cost increases, and the deflection characteristics of the dielectric multilayer film and the flatness of the spectral distribution deteriorate.
When a metal film is to be used, the deflection characteristics and the flatness of the spectral distribution are relatively good, but the film undesirably absorbs light. As a result, the optical characteristics of light amount splitting between transmitted light and reflected light deteriorate.
With the use of the conventional half mirror surface manufacturing techniques, it is difficult to obtain a half mirror that can satisfy both the requirements for optical characteristics and the manufacturing method.